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The older brother of controversial billionaires Charles and David Koch is a kook who spent millions on an elaborate renovation of his lavish Upper East Side townhouse — but instead sleeps in a Fifth Avenue apartment, according to a new book excerpt.

Meanwhile, Frederick “Freddie” Koch, 80, is so cheap that he berates his servants for wasting postage stamps and once held up traffic so he could pry a nickel out of the asphalt in a Manhattan crosswalk, according to the excerpt in Vanity Fair magazine.

The mag’s excerpt from the new book “Sons of Wichita: How the Koch Brothers Became America’s Most Powerful and Private Dynasty” describes Freddie as a mama’s boy who didn’t fit in with Charles, David or fourth brother Bill when they were all growing up.

“They just didn’t want Freddie’s name brought up,” a family friend said.

“They knew there was something different about him. You didn’t hear much about Freddie at all…It was almost like he wasn’t part of the family.”

A new biography “Sons of Wichita: How the Koch Brothers Became America’s Most Powerful and Private Dynasty” reveals how Frederick Koch was ostracized from his family growing up.Photo: WireImage

In 1983, Freddie and Bill netted $800 million by selling their stock in the family’s Koch Industries energy business — then sued Charles and David for allegedly cheating them on the deal, but lost at trial.

Charles and David still control Koch Industries and are reviled by liberals for bankrolling the Tea Party, fighting ObamaCare and challenging climate-change science.

Bill, meanwhile, runs his own energy company and is renowned for winning the America’s Cup yacht race and filing lawsuits over his purchase of counterfeit wine, including several bottles purportedly owned by Thomas Jefferson.

Freddie’s estranged from his younger brothers, but occassionally runs into fellow Manhattanite David at galas and charity events — resulting in “short, awkward exchanges,” the report says.

The report also claims it was “an open secret” in their hometown that Freddie was gay, with sources saying so on and off the record.

During a rare interview in January 2013, Freddie denied being gay — and the story about the nickel — to author Daniel Schulman.

But he gave Schulman a tour of his seven-story townhouse at 6 East 80th St., which was built in the early 1900s by five-and-dime store tycoon Frank Winfield Woolworth for his youngest daughter, Jessie.

Freddie bought the place for $5 million in 1986, then restored it over the course of the next decade.

The work included extending a marble balustrade through the top six stories, replacing the plaster walls with French limestone and widening a pair of stone columns to fit a classic, 19th-century painting, “The Abduction of Psyche” by William-Adlolphe Bouguereau.

“Sotheby’s for years has been trying to take this away from me,” he said of the masterpiece. “And I keep telling them, ‘I can’t move it. It’s part of the architecture of this room.”

Freddie refused, however, to “delve into Koch world” because Schulman wouldn’t sign a contract requiring Freddie’s approval over “all writing” relating to him.